Estrogen causes the formation of spines and excitatory synapses in hippocampal neurons in vitro and in vivo, but the underlying mechanisms are not fully understood. Kalirin (Kal) 7 is exclusively localized to the postsynaptic side of excitatory synapses in hippocampal neurons. Expression of exogenous Kal7 increases spine density whereas reduced endogenous Kal7 decreases spine density in hippocampal neurons in vitro. Kal7 is required for synaptic structure and function in CA1 neurons in Kal7 knockout (Kal7KO) mice. My preliminary data show that Kal7 immunoreactivity in hippocampal neurons is regulated by estrogen in vivo and in vitro;this result was confirmed by Western blot. Estrogen was no longer able to increase synapse formation when endogenous Kal7 expression was reduced. These data led to the hypothesis that Kal7 plays a key role in the mechanisms by which estrogen regulates synaptic plasticity. Aim1. To determine how estrogen increases Kal7 expression in the hippocampus. Estrogen may act directly on pyramidal neurons and/or through interneurons to increase spine/synaptic density in CA1 pyramidal neurons. It is not yet clear whether estrogen regulates Kal7 expression at the transcriptional (mRNA) or post-transcriptional level. Aim1 is to determine: (1) the time course over which estrogen affects Kal7 mRNA levels;(2) whether Kal7 expression changes during the estrous cycle;(3) whether endogenous estrogen affects Kal7 expression;(4) which estrogen receptor (1 or 2) plays a key role in this process. Aim 2. To determine whether Kal7 is essential for estrogen-mediated spine formation and synaptic functions in hippocampal neurons. Understanding the mechanisms through which estrogen regulates Kal7 expression will facilitate manipulation of estrogen-mediated synaptic plasticity. If Kal7 is essential for estrogen-mediated spine formation and synaptic functions, estrogen replacement-mediated spine formation and synaptic functions in ovariectomized (Ovx) Kal7KO mice are likely attenuated or abolished compared to OVX wildtype controls. Aim 2 is to determine: (1) whether Kal7 plays an essential role in estrogen-mediated synaptic function in vitro;(2) whether Kal7 is essential for estrogen-mediated spine formation in vivo;(3) whether Kal7 is essential for estrogen-mediated LTP induction;(4) the pathway through which estrogen regulates Kal7 expression. PUBLIC HEALTH RELEVANCE: Estrogen causes the formation of dendritic spines and excitatory synapses in hippocampal neurons in vitro and in vivo, but the underlying mechanisms are not fully understood. The aim of this research is to determine the effects of Kal7 on estrogen-mediated spine plasticity in hippocampal neurons.